This invention relates to a speech decoder for high quality decoding a speech signal which has been transmitted at a low bit rate, particularly at 8 kb/sec or below.
A well-known speech decoder concerning frames with errors, is disclosed in a treatise entitled "Channel Coding for Digital Speech Transmission in the Japanese Digital Cellular System" by Michael J. McLaughlin (Radio Communication System Research Association, RC590-27, p-p 41-45). In this system, in a frame with errors, the spectral parameter data and delay of an adaptive codebook having an excitation signal determined in the past are replaced with previous frame data. In addition, the amplitude in a past frame without errors is reduced in a predetermined ratio to use the reduced amplitude as the amplitude for the current frame. In this way, a speech signal is reproduced. Further, if more errors than the predetermined number of frames are detected continuously, the current frame is muted.
In this prior art system, however, the spectral parameter data in the previous frame, the delay and the amplitude as noted above are used repeatedly irrespective of whether the frame with errors is a voiced or an unvoiced one. Therefore, in the reproduction of the speech signal the current frame is processed as a voiced one if the previous frame is a voiced one, while it is processed as an unvoiced one if the previous frame is an unvoiced one. This means that if the current frame is a transition frame from a voiced to an unvoiced one, it is impossible to reproduce a speech signal having unvoiced features.